How to change the duration of an added game object in multiple empties simultaneously?
Assign the "template empty" to a new group "Plane.Adder".
Move the "template empty" to an hidden layer, or scene (does not need to be active), or file (and link the group from there).
Add any number of instances of group "Plane.Adder" -> Plane.Adder.001, Plane.Adder.002 ...


When you change the duration at the "template empty" the instances inherit this change.